The cover glass is often formed as a coplanar sheet. As, to image the object by means of the imaging optics, light beams pass from the object through the cover glass, the difficulty arises that these light beams are refracted to different degrees depending on the medium (e.g. air, liquid, etc.) present on the outside of the cover glass. Naturally, this affects the imaging quality.
If the endoscope is formed as an endoscope with variable viewing direction, the refraction of the light beams additionally depends on the viewing direction.